a) Field of the Invention
This invention relates to a method of forming a U-shaped strap for a pulley block having a pair of opposed cheeks and a sheave rotatably mounted therebetween. This invention also relates to a strap formed by the method, and to a pulley block when incorporating such a strap.
Though a pulley block of the kind with which this invention is concerned may be used in a variety of circumstances, it has a particular application in relation to racing dinghies. As such, the invention will be described exclusively with reference to pulley blocks for that use, even though the invention is not to be regarded as limited thereto.
b) Description of the Prior Art
Pulley blocks are used extensively on the running rigging of racing dinghies, to assist in the hoisting and trimming of the sails, as well as for other lines used on dinghies. Such pulley blocks are required to have very low friction in order to minimise losses and also to have relatively low weights. Further, pulley blocks used on racing dinghies are frequently highly stressed and so must display great strength in order not to fail at inconvenient times.
A common form of pulley block has a body defining a pair of opposed cheeks between which is rotatably mounted a sheave, the cheeks being held apart at the required spacing by both a boss within the bore of the sheave and a bridging member external to the sheave. A generally U-shaped metallic strap having a base section from which extends a pair of arms is fitted to the body so that each arm overlies the external surface of a respective cheek, the base section of the strap extending over the bridging member. The strap is usually secured to the body by means of at least one through-fastener, which also serves as a shaft on which the sheave is mounted by suitable bearings. The part of the strap extending over the bridging member is adapted for direct or indirect connection to some other component with which the block is to be associated, such that the load imparted to the sheave by a line passing therearound is transferred through the shaft to the strap, and then to that other component.
Recently, blocks have been introduced which have an annular sheave supported by races defined between the outwardly-directed faces of the sheave and the inwardly-directed faces of the cheeks. As such, there is no need to provide a shaft for rotationally mounting the sheave, so that the load is transferred indirectly from the sheave to the strap, through the cheeks. As a consequence, it has been possible to shorten the length of the arms of the strap, but the attachment of the strap to the cheeks must be enhanced, by providing a multiplicity of fasteners extending through the arms of the strap, adjacent but within the inner periphery of the sheave or through the bridging member. These measures can allow the achievement of a reduction in weight of a pulley block as compared to a more conventional construction.
A pulley block of the kind described above is usually connected to some other component by using a metal shackle engaged with the block strap, which increases the weight of the overall assembly. There is a significantly high line-load between the shackle pin and the base section of the U-shaped strap and this can lead to deformation of the strap, which in turn will affect the performance of the block, when in use. In order to eliminate the need for a shackle, frequently a block will be tied by a cord to some other component and to minimise the likelihood of the cord being damaged by the strap, it is known to give the strap a concave cross-section, when viewed externally of the block. The rounded surface of the strap increases its rigidity and also reduces the angle through which a cord is forced when under load so reducing the breaking force on the cord, but it has the effect of reducing the contact area between the strap and a shackle pin to a point-contact, increasing force on the strap at the interface between the strap and a shackle.
The U-shaped strap is conventionally formed by cutting a strip from a sheet of metal such as stainless steel, usually by a stamping operation which also will form the required holes through the strap. The strap is then placed in a press and formed to have the required finished profile. Though this may reduce the sharp edges along the length of the strap, nevertheless those edges are still likely to damage a cord used to tie the block to some other component.